Purpose: To determine the mechanism of pressure lowering of non-penetrating glaucoma surgery (NPS). Conventional thought indicates that the trabecular meshwork is responsible for the abnormal intraocular pressure (IOP) in glaucoma. If NPS does not affect the meshwork, how could it lower IOP?
Methods: Using human tissue, Schlemm's canal was unroofed using microdissection, and the dissected site studied with scanning electron microscopy. In another experiment, a 30 g cannula was introduced into the canal, and Healon injected into the canal to mimic viscocanalostomy. Microscopic sections were made from both the region of the cannula and distal regions which received only Healon.
Results: Unroofing the canal caused inadvertent tears in the inner wall endothelium of the canal, often exposing the underlying juxtacanalicular tissue (JCT). In the viscocanalostomy experiment, both cannula entry into the canal and Healon injection caused ruptures of the inner wall of the canal, especially anteriorly, creating a channel for aqueous to bypass the JCT and inner wall of the canal, gain direct access to the canal.
Conclusions: NPS is a clever and gentle way to perform a "canal-otomy," allowing aqueous to enter the canal through microruptures of the wall of the canal wall and bypass the suspected sites of outflow resistance in glaucoma.